ROVlDED. Jntil recently, most humans identified with WNV infection presented with symptoms of WNV fever or meningoencephalitis. By the time symptoms appear, viremia has typically cleared and immune responses evolved, making it difficult to accurately characterize the clinical spectrum of infection or study the interplay between the virus and the immune system. This situation changed dramatically when WNV RNA screening of blood donors using nucleic acid amplification tests (NAT) was implemented in the U.S. in mid-2003. NAT screening will allow characterization of the dynamics of acute viremia and time course of immune responses in primary WNV infection. Prospective enrollment of viremic, pre-seroconversion donors and detailed clinical, virologic and immunologic studies will better define the natural history of infection and the viral and host factors that correlate with control of viremia and varied WNV disease syndromes. We will draw our samples from a unique cohort of viremic individuals detected by the .American Red Cross (ARC) and Blood Systems (BSI) networks of blood banks, which together collect approximately 60% of the U.S. blood supply. Medical staff at ARC and BSI will administerinterviews (developed in collaboration with CDC) to ascertain the rates of development and severity of WNV-symptom complexes. A subset of viremic donors who develop severe symptomatic infections and matched asymptomatic controls, will be enrolled into a sub- study. We will perform virologic and immunologic analyses of frequent sequential samples to characterize the kinetics of primary viremia and the time course of WNV-specific IgM, IgA, IgG and plaque neutralizing antibody seroconversion. Detailed studies of T cell responses, including kinetics and antigen specificity, and of cytokine perturbations will be carried out on the same cases. The findings from these studies will therefore address key blood safety issues and contribute to our understanding of the natural history and pathogenesis of WNV infection by identifying viral and humoral immune parameters that correlate with control of viremia.